Lezione 13: Materials at high temperature Flashcards
What are viscous flow and creep?
Flow rate in fluids under a shear stress depends on the fluid viscosity. Creep is a slow, continuous deformation of a material at elevated temperatures, ending in fracture
How is creep tested?
The specimen is loaded in tension or compression, usually at constant load, inside a furnace that is maintained at a constant temperature.
What’s the use of a stress-rupture curve?
Design data based on creep is generally presented in a stress-rupture curve. It allows you to identify either the design stress or rupture life at a given temperature
What is the melting point importance?
The temperature at which a material starts to creep depends on its melting point. Polymers actually can start to creep at room temperature.
What is the use of the strength-maximum service temperature graph?
At room temperature, material selection requires only a single strength-density chart. For high temperature design, charts are needed that account for temperature and an acceptable strain rate
What are the consequences of creep?
Creep fracture: diffusion can cause creep as well as fracture due to creep by creating voids that nucleate on grain boundaries
Creep in polymers: as in crystalline solids, polymers creep and creep is often related to diffusion. Diffusion requires free volume, which is found dispersed among all atoms in a polymer (since we have no lattice structure). Free volume increases with T and does so most rapidly at Tg. Polymers behave in a visco-elastic manner around their Tg, meaning they act neither like an elastic solid or viscous liquid. This nature reduces the rate of creep while being loaded and allows for a small amount of reverse creep upon unloading. The creep modulus Ec is used when designing polymers against creep.
What are thermal barrier coatings?
Design against creep an include the use of thermal barrier coatings. For a turbine blade, a ceramics coating is applied to the metal surface allowing for an increase in gas temperature with no increase in that of the blade itself.
